Blood flow rate, tube length, and ultrafiltration are demonstrated to influence tubular platelet concentration in a capillary tube during flow from a reservior. Citrated or hepainized bovine or human blood (hematocrit is equal to 35) is forced to flow through a tube (polyvinyl chloride, polysulfone or siliconized glass) because of inflow to the stirred reservior from a syringe pump. A continuous saline drip carries outflow blood from the tube exit to a collection flask. Accurate dilutions are made of blood collected from the reservior, tube and outflow; samples are electronically counted to determine the platelet concentrations. Platelet concentrations in the reservior and collected outflow are equal, thus indicating that platelet reaction is not a factor; instead, flow-induced red cell motions apparently reduce the average platelet velocity. At moderate wall shear rates (400 sec to the minus 1st power) in 250 micron tubes that are 9-12 cm long and have no ultrafiltration, the ratio of tube platelet concentration to reservoir platelet concentration is 1.2. In a 2 cm tube at the same conditions, the ratio is 1.7. At high wall shear rates, no elevation in tubular platelet concentration is measured. For moderate wall shear rates with an ultrafiltrative flow of half the inlet flow, a ratio of up to 5 is obtained. Primary extensions of the research are to other tube diameters and hematocrits.